Reduced gas holdup in catalytic reactor

ABSTRACT

A reactor for refining resid includes an elongated vertical vessel containing a bed of catalyst which is expanded or ebullated by a liquid/gas mixture. The mixture rises in an updraft through the bed and then is recirculated with an increased velocity by an ebullating pump. To reduce gas holdup of the ebullated bed and to promote a more uniform flow, the gas is entrapped by two cascaded stages of baffles which have an internal conical shape to guide, direct, and entrap the gas. One of these two stages has a shape which creates a countercurrent in the flowing liquid to deflect any catalyst particles which might otherwise be elutriated by the increased velocity.

This is a division of application Ser. No. 533,388, filed Jun. 5, 1990which is a continuation in part of Ser. No. 249,605, filed Sep. 26,1988, now U.S. Pat. No. 4,950,459, which in turn is a divisional of Ser.No. 087,394, filed Aug. 20, 1987, now U.S. Pat. No. 4,804,458.

This invention relates to catalytic reactors and more particularly toreactors with a reduced gas holdup in an expanded catalytic bedused--especially, but not exclusively--for upgrading resid.

For purposes of this disclosure, the term "vapor", refers to andcomprises excess hydrogen, light hydrocarbon gases, hydrogen sulfide,ammonia, steam, etc. emitted in the reaction zone.

Reference is made to U.S. Pat. No. 4,804,458 which shows two of thethree stages described herein. Much information which is described indetail in this patent may complete the information set forthhereinafter. A number of other patents show reactors and processes whichhave been suggested for processing oil. Typifying these prior artreactors and processes are those found in U.S. Pat. Nos. 3,124,518,3,227,528, 3,414,386, 3,677,716, 4,057,397, 4,097,243, 4,221,653, andRe. 25,770. These prior art reactors and processes have met with varyingdegrees of success.

During hydrotreating, residual oil ("resid") is upgraded with hydrogenand a hydrotreating catalyst in a three-phase mixture of oil, catalyst,and vapor or gas bubbles to produce more valuable, lower-boiling liquidproducts. The gas phase (hydrogen) is required in some minimum quantityin order to carry out the catalytic refining process within the reactor.

In order to increase the efficiency, effectiveness, and profitability ofresid hydrotreating, it is desirable to maximize the conversion of residinto more valuable lower boiling liquid products. The extent of theconversion of resid into these more valuable lower-boiling liquidproducts depends in part on the residence time of the resid inside thereactor which, in turn, depends upon the effective volume of the reactoritself.

In greater detail, a reactor has a very large, sealed vessel or chambercontaining a bed of catalytic particles. In ebullated (expanded) bedreactors, the reactor oil and catalyst bed are fluidized, ebullated, andexpanded. The oil and gas rising within the chamber lifts and expandsthe catalyst bed. However, the gas rising through the bed also occupiesspace which tends to reduce the liquid volume available for upgradingthe resid. If excessive amounts of the gas phase (vapor) is entrained inthe recycled reactor oil it can lead to a high internal recirculation ofgas which can cause an even higher gas holdup. This further reduces theliquid volume for thermal reactions within the reactor. Therefore, anydecrease in gas volume within the reactor increases the residence, ordwell time, of the resid within the reactor.

As described thus far, the reactor oil does not usually have enoughvelocity to properly expand the catalyst bed above its settled level;therefore, some of the oil is recycled within the reactor. Moreparticularly, a portion of the rising oil overflows and collects in arecycle pan from where it falls through a downcomer or pipe and thenreturns to the bottom of the catalyst bed under the force imparted by anebullating pump. The rising oil entrains some gas as it enters therecycle pan. The relatively large volume of the recycle pan decreasesthe velocity of the recycle oil and allows some entrained gas bubbles toescape. However, some of the entrained gas remains in the recycleliquid.

The U.S. Pat. No. 4,804,458 provides a second stage in the form of afrustroconical skirt, under the recycle pan, in order to entrap and ventthe entrained gas before it can reach the downcomer and be recirculated.The present invention adds a third stage which is a second skirt thatentraps and vents even more of the entrained gas before it can reach thedowncomer. In order to minimize gas entrainment in the recycle liquid,vapor or gas bubbles in the reaction zone are trapped and caught in bothof the skirts at a predetermined positions below the upper surface ofthe liquid (i.e. below the liquid level). The entrapped gas is directedor injected into a vapor containing space above the liquid level.

It has been found that there is a close relationship between reactorliquid volume and reactor gas holdup. More particularly, residconversion is a thermal reaction, with the catalyst serving to stabilizethe cracked products. Since it is a thermal reaction, the residconversion increases with an increase in temperature and with anincrease of the dwell or residence time of the liquid within thereactor. Since the residence time varies directly with liquid volumewithin the reactor, an increase in the liquid volume (decrease in gasvolume) increases the amount of resid conversion. The Ramscarbonconversion also increases because the reaction of coke precursorsdepends strongly upon thermal reactions. In a large refinery, thisamount of increased resid and Ramscarbon conversion can bring aneconomic advantage which may easily exceed $1.5 million per year attoday's prices.

Hence, there is a dilemma since a certain minimum mass rate of gas isnecessary, but excess volume of gas is undesirable. There is a constantneed for reducing gas holdup without sacrificing the mass rate of gasthat is available for necessary upgrading reactions.

Another point is that the rising gas creates a very strong updraftzone(s) within the ebullated catalyst bed. Catalyst particles becomeentrained in these zone(s), and a low concentration of catalystparticles exists above the catalyst bed proper. Entrained or elutriatedparticles can result in slow but persistent flow of catalyst down therecycle line. Some of these particles may also pass out of the reactorwith the refined product. Among the unfavorable effects of suchelutriation are a break up of the catalytic particles in the recyclepump or loss of inventory control of the catalyst.

Therefore, another purpose of the inventive third stage in the reactoris to create flow currents around the lower skirt which have the effectof blocking particle entrainment and the resulting elutriation.

Accordingly, an object of the invention is to provide new and improvedmeans for and methods of reducing gas holdup in an expanded catalyticbed.

Another object of the invention is to create flow currents under therecycle pan which greatly reduce particle entrainment in the oil andelutriation from the reactor and return entrained catalyst particles inthe freeboard to the catalyst bed.

Yet another object is to maintain a more uniform inlet pressure for therecycle pump. In this connection, an object is to stabilize the recyclepump flow.

In keeping with an aspect of the invention, the novel ebullated bedreactor has a feed line which feeds a mixture comprising oil andhydrogen-rich gases into the plenum of the reactor. The feed iscirculated within the vessel and the catalyst bed is expanded by anebullating pump located in the lower portion of the reactor. A tubulardowncomer extends generally upward from the ebullating pump and into thefreeboard zone. A stationary recycle pan in the upper portion of thevessel is connected to the upper end of the downcomer in order toseparate vapor from the recycle reactor oil flowing into the downcomer.An annular skirt is connected to the bottom of the pan and extendsdownwardly and outwardly therefrom for collecting vapor and gas in theupper portion of the freeboard zone.

Beneath the annular skirt is a second skirt in the form of a toroid witha beveled roof for collecting raising vapor or gas which may be in theoil. Between the downcomer and the internal periphery of the second andtoroidal skirt, there is a first and relatively large annular gap. Asecond and smaller annular gap is at the external periphery of thetoroidal skirt which is close to the reactor wall. Since the first gapis much larger than the second gap, and in view of the shape of theskirts, a circulatory liquid flow pattern develops around the secondskirt. The circulating liquid entrains catalyst particles and returnsthem to the catalyst bed along a path extending downward through thesecond annular gap.

A more detailed explanation of the invention is provided in thefollowing description and the appended claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of resid hydrotreating units and associatedrefinery equipment for carrying out the process;

FIG. 2 is a cross-sectional view of the ebullated bed reactor equippedwith a two stage vapor collector in accordance with the teachings ofU.S. Pat. No. 4,804,458;

FIGS. 3 and 4 are enlarged partial-cross sectional views of otherembodiments of the vapor collector in accordance with the principles ofU.S. Pat. No. 4,804,458;

FIG. 5 is an enlarged view of the reactor of FIG. 2 which is useful forexplaining the third reactor stage;

FIGS. 6 and 7 are graphs showing the comparative advantages of the one,two, and three stages of the invention;

FIGS. 8 A and B are two graphs taken from tests conducted in a columnwhich was four feet in diameter charged with catalyst, nitrogen, andkerosene to indicate the comparative advantages of the one, two, andthree stages of the invention; and

FIG. 9 shows the pertinent dimensions of the column used to run thetests reported in FIGS. 8A, 8B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

High-sulfur resieual oil feed, also called vacuum-reduced crude,comprising 1,000+° F. resid and heavy gas oil, is fed into residhydrotreating units (RHU) 7, 8, 9 (FIG. 1) along with a hydrogen-richfeed gas. Each resid hydrotreating unit is a reactor train comprising acascaded series or set of three ebullated (expanded) bed reactors 10,12, and 14. In the reactors, the resid is hydroprocessed (hydrotreated)in the presence of fresh or equilibrium hydrotreating catalyst andhydrogen to produce an upgraded effluent product stream with reactortail gases (effluent off gases) leaving used spent catalyst.Hydroprocessing in the RHU includes demetallation, desulfurization,denitrogenation, resid conversion, oxygen removal (deoxygenation), andremoval of Ramscarbon.

The resid hydrotreating units and associated refining equipment (FIG. 1)comprise three identical parallel trains of cascaded ebullated bedreactors 10, 12, and 14, as well as hydrogen heaters 16, influent oilheaters 18, an atmospheric tower 20, a vacuum tower 22, a vacuum toweroil heater 24, a hydrogen compression area 26, oil preheater exchangers28, separators 30, recycled gas compressors 32, air coolers 34, raw oilsurge drums 36, sponge oil flash drums 38, amine absorbers and recyclegas suction drums 40, and sponge oil absorbers and separators 42.

Each of the reactor trains comprises three ebullated bed reactorscoupled in series. The oil feed typically comprises resid oil and heavygas oil. The feed gas comprises upgraded recycle gases and fresh makeupgases. Demetallization primarily occurs in the first ebullated bedreactor in each train. Desulfurization occurs throughout the ebullatedbed reactors in each train. The effluent product stream typicallycomprises light hydrocarbon gases, hydrotreated naphtha, distillates,light and heavy gas oil, and unconverted hydrotreated resid.

The resid hydrotreating unit is quite flexible. If desired, the samecatalyst can be fed to one or more of the reactors or a separatedemetallation catalyst can be added to the first reactor while differentcatalysts can be added to the second or third reactors. Alternatively,different kinds of catalysts can be fed to each of the reactors ifdesired. Typically, the used and spent catalyst contains nickel, sulfur,vanadium, and carbon (coke). Many tons of catalyst are transported into,out of, and replaced in the ebullated bed reactors daily. Hence, thecost and preservation of catalyst becomes an important economic factorin the refinery operation.

The products produced from the resid hydrotreating units in theebullated bed reactors include light hydrocarbon gases, light naphtha,intermediate naphtha, heavy naphtha, light distillate, mid-distillate,light gas oil, vacuum naphtha, light vacuum gas oil, heavy vacuum gasoil, and hydrotreated vacuum resid. Light and intermediate naphthas canbe sent to a vapor recovery unit for use as gasoline blending stocks andreformer feed. Heavy naphtha can be sent to the reformer to producegasoline. The mid-distillate oil is useful for producing diesel fuel andfurnace oil, as well as for conveying or cooling the spent catalyst.Light and heavy vacuum gas oils and light gas oil are useful asfeedstock for a catalytic cracker. The vacuum resid can be sent tocokers to produce coke.

The ebullated bed reactor 10, as illustrated in FIG. 2, is an elongatedvessel 44 with an upright wall 45, having a lower portion 46 and anupper portion 48 with a top, roof or dome 49. An oil feed line 50,conduit or pipe, provides an oil feed comprising resid (resid oil) andhydrogen-rich gases into the reaction zone 52 in the lower portion 46 ofthe vessel 44. The top surface of the liquid (reactor oil) in thereactor 10 is called the liquid level 54. A catalyst feed line, conduitor pipe 56 feeds fresh or replacement hydrotreating catalyst into theupper portion 48 of the reaction zone 52 to provide a catalyst bed 58 inthe vessel 44. A spent catalyst outlet, withdrawal line, conduit or pipe66 withdraws spent catalyst from the lower portion 46 of the vessel 44.The catalyst bed 58 can be expanded from a settled bed level 60 to anexpanded catalyst bed level 62. Part of the inventive structure is toprevent entrained catalyst particles from being elutriated out of thebed.

The upgraded oil produced in the vessel 44 is withdrawn through an upperproduct outlet pipe or withdrawal line 64. Alternatively, a lowerproduct outlet, pipe or withdrawal line 65 can withdraw the product nearthe surface of the liquid level 54 and discharge the product out of thereactor near the lower portion 46 of the vessel 44. The lower productoutlet pipe 65 can extend downwardly along the upright wall 45 of thevessel 44 in the reaction zone 52 through the distribution plate 72.

Ebullated bed reactors have many advantages over fixed bed reactors.They enable an operation at higher average temperatures and permit theaddition and withdrawal of catalyst without requiring a reactorshutdown. They avoid plugging due to dirty feed and help minimizeformation of solids during resid conversion. Ebullated bed reactors maybe operated at extremely high temperatures and pressures.

An ebullating pump 68 is located in the lower portion 46 of the vessel44, for controlling, expanding and varying the height of the catalystbed 58 as well as for circulating the oil. More particularly, since theliquid resid feed does not usually have enough velocity to expand thecatalyst bed above its settled level, liquid is recycled from the upperportion 48 of the reactor 10 to the bottom of the reactor 10 through avertical downcomer, pipe or recycle line 78. Then, the recycled liquidis pumped through the reaction zone 52 of the reactor 10 at a velocitysufficient to attain the required degree of expansion in order to keepthe catalyst bed fluidized. Liquid recycle flow rates and the catalystbed 58 level in reactors are controlled by varying the speed of theebullating pump 68 which can range from about 400 to 1,800 RPM.

An elongated stationary tubular downcomer 78 extends generally upwardabove the ebullating pump 68 to recirculate oil after it has risenthrough the catalyst bed. The downcomer 78 has an upper end portion 80and a lower end portion 82. The upper end portion 80 of the downcomer 78also has an inner surface 84 and outer surface 86.

The lower portion 46 of the reactor includes a bottom section 70 with ahorizontal distributor plate 72 which separates the reaction zone 52from the bottom section 70 of the reactor 10. The distributor plate 72has a multitude of bubble caps 74 and risers 76 which direct the flow ofoil and hydrogen-rich gases upwardly and into the reaction zone 52,while preventing a flow of the catalyst downwardly into the bottomsection 70.

A recycle pan 88 serves as one stage of the vapor separator in order tohelp disengage or separate the hydrogen gas and other vapors from theliquid or oil feed entering the downcomer 78. The large diameter of therecycle pan 88 decreases the velocity and motion of the oil, making iteasier for gas vapors and bubbles to escape entrainment. The internallyrecycled oil feed continues through the ebullating pump 68 and throughthe reaction zone 52.

The recycle pan 88 is fabricated of a metal, such as stainless steel,which resists corrosion from the oil feed, gases and products producedin the reactor 10 and which substantially maintains its structuralintegrity and strength at hydrotreating conditions. The recycle pan 88(FIGS. 2 and 3) has an inner centrally located feed passageway 90 and isgenerally funnel-shaped with a substantially circular upright wall 92having a top portion 94 and bottom portion 96. A recycle pan wall 92helps to achieve a more uniform liquid flow profile inside the pan 88.The pan 88 has an inwardly sloped flared lower pan wall 98 (FIG. 3),with an inner surface 100 and an outer surface 102, comprising anannular inwardly sloping frustoconical flared wall extending downwardlyand inwardly at an angle of inclination from the bottom portion 96 ofthe upright wall 92. The sloping wall 98 extends downwardly and inwardlyat an angle of inclination ranging from about 30° to 60° in relation tothe vertical axis, and preferably from about 40° to 50°. A bottom edge104 of the sloping pan wall 98 is attached to and above upper endportion 80 of the downcomer 78.

High gas entrainment within oil recirculating through downcomer 78causes high ebullating pump speeds. That is head requirements for theebullating pump 68 rise rapidly with increased gas entrainment because,with more gas present, the pump inlet pressure falls due to the lowerhydrostatic head pressure in the downcomer 78. To counteract the lowerinlet pressure, the centrifugal ebullating pump 68 has to increase itsspeed to deliver the recirculating oil with enough pressure to drive itthrough the catalytic bed.

As a second stage gas/oil separator, a stationary flared annularfrustoconical rigid skirt 106 (FIG. 2) is fixedly attached to the bottomedge 104 of the upright wall 92 of the recycle pan 88, and extendsdownwardly and outwardly therefrom to a position spaced inwardly fromthe upright wall 45 of the vessel 44. The skirt 106 is fabricated of ametal, such as stainless steel, which resists corrosion from the oilfeed, gases and products produced in the vessel 44 and whichsubstantially maintains its structural integrity and strength athydrotreating conditions.

The skirt 106 extends downwardly and radially outwardly at an angle ofinclination ranging from about 5° to 45° in relation to the verticalaxis, preferably from 5° to 25°, and most preferably at 20° in relationto the vertical axis for best results. Since this angle far exceeds thecatalyst friction angle, catalyst should freely slide off the uppersurface 114 of the skirt 106. The skirt 106 has a smaller outsidediameter than the inside diameter of the upright wall 45 of the vessel44 so as to provide an annular passage or gap 107 therebetween.

The annular passageway or gap 107 must be large enough to allow theliquid (reactor oil) to freely flow, circulate and pass upwardly throughthe passage 107 while reciprocatingly allowing large clumps of catalystor coke to pass downwardly through the passage 107 and return to theexpanded catalyst bed level 62. Test results indicate that decreasingthe span (width) and cross-sectional area of the passage 107 byenlarging and increasing the width and cross-sectional area of the skirt106, improved gas separation performance and reduced gas holdup aroundthe pan 88 by as much as a factor of two.

One reason for the performance improvement with the decreasing of thepassage 107 width is the change in the projected area of the skirt 106and Vapor collector 105. With a smaller passage, the skirt 106 and vaporcollector 105 occupied a larger cross-sectional area, increasing itscapability of capturing gas and conveying it to the conduits 118.Another reason for the improved performance was the "channeling" effectwhere the narrow passage 107 directed ga to a channel along the reactorvessel wall 45 where it could more readily bypass the recycle pan 88.

The minimum passage 107 width between the inner reactor wall 45 and theouter edge of the skirt 106 is a compromise between a desire to improvegas disengagement and a desire to minimize the risks associated withcatalyst and coke deposits. The passage 107 must be large enough toprevent large clumps of catalyst or coke to wedge into the annulusformed by skirt 106 and the vessel wall 45. The preferred width of thepassage 107 in one reactor was nine inches, to minimize thispossibility.

Increasing the depth of the skirt 106 provides a vapor collector 105with a greater cross-sectional area and greater volume. Test resultshave shown that a deeper skirt 106, such as from 10 inches to 14 inches,reduces by 4 to 6 volume percent the amount of gas, vapor, and bubblesthat may enter into the downcomer 78. With a larger cross-sectionalarea, the skirt 106 and vapor collector 105 provide a larger pocket 116for the capture and collection of vapor (gas bubbles). A deeper skirt106 and vapor collector 105 also provide more residence time for thedisengagement or separation of gas and liquid.

Gas and vapor separation improved significantly as the skirt 106 becamedeeper and wider. However, there are mechanical constraints. If theskirt 106 depth extends to and makes contact with the expanded catalystbed level 62, an accumulation of coke, asphaltenes, catalysts, and othersolids can adhere to the skirt 106, lowering the efficiency of theebullated bed reactor and hydrotreating process. Extending the skirtinto the catalyst also makes it difficult to control the expandedcatalyst bed level 62.

The skirt 106 (FIG. 3) has an upper portion 108 and a lower portion 110.The slanted span of the skirt 106 can be about 24 inches wide. The skirt106 has a downwardly facing bottom skirt surface 112 and an upwardlyfacing upper surface 114. The bottom surface 112 of the skirt 106provides a deflector and a baffle for deflecting the rising gas bubblesor vapor in the reaction zone 52. The bottom surface 112 of the skirtcooperates with the inwardly sloping wall 98 of the recycle pan 88, todefine an annular inverted V-shaped bubble-receiving pocket 116 forcatching, trapping, and receiving a substantial amount of vapor and gasbubbles entrained in the oil feed and deflecting the collected vapor andgas bubbles upwardly through vapor risers or vertical conduits 118.

The vapor collector 105 and recycle pan 88 provide a two-stage separatorto reduce gas entrainment in the recycle liquid (reactor oil) and gasholdup in the reaction zone 52. The vapor collector stage 105, collects,pockets, disengages and separates the vapor and gas bubbles in theliquid (reactor oil) above the reaction zone 52. The recycle pan stage88 further disengages and separates the vapor and gas bubbles from theoil feed inside and in proximity to the recycle pan 88.

The vapor collector and assembly 105 include: (1) the flared annularfrustoconical skirt 106; (2) the inwardly sloping wall 98 of the recyclepan 88; (3) the annular inverted bubble-receiving pocket 116 between theskirt 106 and the pan wall 98 for catching, trapping, and receivingvapors and gas bubbles in the oil feed and reaction zone; and (4) thevertical conduits 118 which provide a passageway for transporting orventing the collected vapor and gas bubbles away from the pocket 116into the vapor space 124 at the top 49 (FIG. 2) of the vessel 44.

Test results indicate that the skirt 106 and vapor collector 105 reducedgas holdup around the recycle pan 88 by a factor of two or more. As theskirt 106 and wall 45 of the vessel 44 became narrower, more gas flowedthrough the conduits 118, and gas holdup dropped around the recycle pan88.

Reducing gas holdup around the pan 88 benefitted separation in two ways.As gas holdup around and inside the recycle pan 88 declined, bubbles,and vapors rose faster increasing the effectiveness of the separationinside the recycle pan 88. The other effect is related to gasconcentration. Lower gas concentration or holdup means that there isless gas to entrain. Thus, there is a correlation between gas holduparound the recycle pan 88 and gas entrainment in the downcomer 78. At aconstant liquid recycle rate, gas entrainment in recycle liquid rosesharply as gas holdup around the pan 88 increased.

The skirt 106 and vapor collector 105 not only reduced gas holdup in thedowncomer (recycle line) 78 and in the region around the recycle pan 88,but it also reduced gas holdup in the reaction zone 52 of the reactor10. Gas holdup declined overall because less gas was recycledinternally. Gas velocities dropped in the upflow region in the reactionzone 52 of the vessel 44.

One series of tests indicated that the skirt 106 and vapor collector 105decreased gas holdup in the reactor zone from about 35 volume percent toabout 30.5 volume percent.

A reduction in reactor gas holdup has significant impact on residconversion. For example, four volume percent reduction in gas holdup isequivalent to a 2 percentage point increase in resid conversion becausethere is an increase in the liquid volume available for thermalreactions. This increased liquid volume also increases Ramscarbonconversion since this reaction also depends on thermal reactions.

Conduit diameter can be an important factor in the design of the vaporcollector 105. If the diameters of the conduits are too large, excessliquid can flow through the conduits 118 impairing gas disengagement,liquid/gas separation and increasing gas holdup. High liquid ratesflowing through the conduits 118 can also transport too much liquid intothe vapor space 124. When liquid rate exceeds the rate of the productwithdrawal through the product outlet line 64, liquid can flow back tothe recycle pan 88 in a direction opposite the gas flow and gasdisengagement.

Calculations showed that less than 20% of the liquid flowed through fourtwo-inch conduits 118. This rate increased to over 40% when the diameterof the four conduits 118 was expanded to three inches. At the same time,calculated gas holdup in the risers dropped from 62 volume percent to 47volume percent.

Each conduit 118 (FIG. 3) can have a lower section 120, an elongatedintermediate section 119 and an optional upper open ended section oradapter 122 for attachment to supports. The lower section 120 has asmaller diameter than the adapter 122. These adapters 122 can compriseshort cups with a diameter larger than the intermediate section 119 ofconduit 118, such as about double that of the intermediate section 119.The adapters 122 can be useful to limit liquid splashing in the vaporspace 124 and thus reducing the potential for reentrainment of the gas.

The lower section 120 (FIG. 3) of the conduit 118 is welded or otherwisesecurely connected to and extends upwardly from the upper portion 108 ofthe skirt 106 to a vapor space 124 above the liquid level 54 (FIG. 2) ofthe oil feed. The conduits 118 pass the vapor and gas bubbles caught inthe pocket 116, to the vapor space 124 in proximity with the top 49 ofthe vessel 44. This minimizes entrainment and gas holdup of gas bubblesin the oil in the reaction zone 52 and increases the effective volume ofthe reaction zone 52 by decreasing internal gas recycle.

The lower section 120 (FIG. 3) of the conduit 118 does not extend belowthe skirt 106, in order to: (1) provide a smoother uninterrupted flowpattern of the oil feed, (2) prevent coking, accumulation ofasphaltenes, catalysts, and other solids near the lower section 120 ofthe conduit 118, and (3) avoid creating an internal cylindrical bafflein the pocket 116 which would occupy valuable reactor volume, causeundesirable deflection, and interrupt the efficiency of the vaporcollector 105.

It was originally expected that a vapor collector 105 without conduitswould perform about the same or worse than reactors equipped with only arecycle pan, (i.e. without a skirt) and without the vapor collector 105.It was unexpectedly and surprisingly found, however, that the vaporcollector 105 without conduits exhibited much better results than areactor equipped with only a recycle pan. Through the addition of theskirt 106, vapor collector 105, and pocket 116, about 8 volume percentless gas entered into the downcomer 78. The passage 107 provided achannel or pathway along the vessel 44 wall 45 where the vapor or gaspromoted channeling of the vapor and allowed the vapor to bypass therecycle pan 88 and flow directly to the vapor space 124 at the top 49 ofthe vessel 44.

Furthermore, it was unexpectedly and surprisingly found that a partialplugging of conduits 118 was not substantially detrimental to gasdisengagement and gas holdup reduction until all of conduits 118 wereblocked because substantial gas disengagement and reduction of gasholdup occurred in the pocket 116 of the vapor collector 105. Ascompared to a vapor collector 105 with two or four open conduits 118,the vapor collector 105 without conduits or with plugged conduitsallowed about 4 volume percent more gas into the downcomer 78.

It was also found that increasing the number of conduits 118 is similarto increasing conduit diameter because the total cross-sectional area ofthe conduits increases. When this area becomes too great, gasentrainment gets worse. Since it is important that the totalcross-sectional area of the conduits 118 be small enough to prevent asubstantial passage of liquids (reactor oil), the number of conduits 118should be increased only if the conduit 118 diameter can be decreased ina compensating manner.

Preferably, the total cross-sectional area of all of the conduits 118should be less than 1.5% of the cross-sectional area of the vessel 44for best results. Each conduit 118 should also have a sufficient innerdiameter to avoid plugging or clogging by catalyst, coke, etc.

While the illustrative embodiments are preferred for the best results,it may be desireable in some circumstances to have: (1) more or lessthan six conduits extending above the skirt; (2) staggering or varyingthe heights of the conduits above the skirt; (3) staggering thedistances by which the conduits are spaced from the center of the skirt;or (4) extending one or more of the conduits below the skirt.

Ribs or reinforcing struts 126 (FIG. 3) and 128 can be utilized tostabilize and secure each conduit 118 to the skirt 106. The lower struts126 are generally triangular and extend between and securely connect thelower section 120 of conduit 118 to the upper portion 108 of the skirt106. The upper strut 128 extends between and securely connects the lowersection 120 of the conduit to the upright wall 92 of the recycle pan 88.

FIG. 4 illustrates another embodiment of the vapor collector 105. InFIG. 4, the upper portion 108 of the skirt 106 includes an annularupright wall 130, which has a diameter that is slightly larger than thediameter of the upright wall 92 of the recycle pan 88, to simplify theretrofitting and attachment of the skirt 106 to recycle pan 88. Theupright skirt wall 130 is positioned against the pan wall 92 in abuttingrelationship. The upright wall 130 of the skirt 106 has an upper portion134. The upper wall portion 132 has a hook or inverted J-shaped finger136 for attaching the inwardly facing surface of the upright skirt wall130 to the exterior surface of the upright pan wall 92. The lowerportion 134 of the upright skirt wall 130 is attached to the upper strut128 for stabilization and enhanced structural strength and integrity.

FIG. 5 is a greatly enlarged view of portions of FIG. 2 for explainingthe construction and operation of the third separation stage. Since manymechanical details have already been explained in connection with theconstruction of skirt 106, and conduits 118, they are omitted from FIG.5 and from this description of a third stage, with the understandingthat except for the differences explained below approximately the samemechanical details are also present in the third stage of FIG. 5.

Briefly in review, a there are three vertically deployed gas separationstages. A first stage for removing gas is provided by recycle pan 88.The principle used in the first stage is that the inside diameter 200 ofthe pan 106 is so large compared to the inside diameter 202 of thedowncomer 78 that the velocity of the oil slows. As the oil velocityslows, the natural buoyancy of the entrained vapor and gas causes themto rise and escape into a vapor space area 124 at the top of the reactorvessel.

The second stage is the frustroconical skirt 106 which is attached tothe bottom of the recycle pan 88. As the processed oil rises in thereactor, the vapor and entrained gas are trapped under the skirt 106.The buoyancy of the vapor and entrained gas causes them to collect atthe top of the area 204 under frustroconical skirt 106. This entrappedvapor and gas escapes from under skirt 106 via a plurality of conduits118 into the vapor space 124 at the top of the reactor 10.

The invention adds a third stage which is a second skirt 206 suspendedbeneath skirt 106 by any suitable mean (not shown) such as rods orstruts welded between skirts 106, 206 and other nearby structuralmembers. The two skirts 206, 106 are constructed of similar materialsand in similar manners, which have already been explained.

The second skirt 206 is a toroid having a bevel roof formed by two sides208, 210 set at an acute angle A, with respect to each other. As withthe skirt 106, the vapor and gas entrained within the oil rises into anarea 212 near the ridge of the beveled roof, where is it entrapped.Again, a plurality of conduits 214 (similar to conduits 118) provide apath for the vapor and gas entrapped in area 212 to escape into thevapor zone 124.

The third stage skirt 206 provides a second function of eliminatingentrained catalyst particles that reach the freeboard area 218 above thetop surface 62 of the ebullated catalyst bed. In greater detail, thesurface 62 at the interface between freeboard oil 216 is usually fairlywell defined.

Some catalyst particles are entrained, however, into the freeboardregion above the catalyst bed. If unrestrained, the particles entrainedin the oil travel with it escaping from the catalyst bed (called"elutriation"). Some particles go through the downcomer 78 and someleave the reactor with the product at exit 64. The particles passingthrough the downcomer 78 reach the ebullating pump 68. The particlespassing out of the reactor make it difficult to maintain a desiredcatalyst inventory within the reactor vessel. Other reasons whyentrained catalyst particles are undesirable will occur to those skilledin the art.

Means are provided for eliminating the entrainment of these particlesand the elutriation resulting therefrom. In greater detail, the toroidalskirt 206 has a relatively long side 208 extending from an outerperiphery 220 to an apex 222 or ridge of the bevel roof. A relativelyshort side 210 extends from an inner periphery 224 to the apex 222 ofthe roof. Thus, the inner periphery or eave 224 of the roof is muchhigher than the outer periphery or eave 220. Also, the inner annular gap226 between the inner periphery 224 and downcomer 78 is much larger thanthe outer annular gap 228 between the outer periphery 220 and the insidesurface wall 45 of the reactor. The width of outside gap 228 is in therange of 1"-18" for a twelve foot interior vessel diameter, with 9"preferred for reasons pointed out above. The dimension of the gap is notcritical if the desired liquid circulation pattern properly establishesitself.

It has been found that with a properly designed and shaped skirt 206almost all of the entrained catalyst particles can be returned to thecatalyst bed, thus eliminating almost all of the elutriation. In greaterdetail, the described construction with inner eave 224 higher than outereave 220, and with an inner annular gap 226 which is larger than theouter annular gap 228, a liquid circulation pattern 230 establishesitself around the toroidal skirt 260. This current 230 does notinterfere significantly with the upward passage of oil to the recyclepan 88 and to the product exit pipe 64. The liquid circulation pattern230 is strong enough so that entrained particles flow downwardly throughgap rather than upwardly through the gap between skirt 106 an vesselwall 45. The circulation pattern developed by skirt 206 reduces theamount of catalyst entering the product line or the recycle line.

Thus, the third stage 206 provides the following benefits:

1) Entrained gas is reduced in order to reduce the gas holdup in thedowncomer and thereby improve the operation of the ebullating pump 68 bymaintaining a more uniform pressure head in downcomer 78.

2) The recycle flow up the catalyst bed, down the downcomer, and backinto the catalyst bed is much more stable and predictable.

3) Gas holdup is reduced in the catalyst bed by reducing gas entrainedand the resulting gas recycle.

4) Catalyst elutriation is greatly reduced from the elutriating levelwhich would normally be expected.

FIG. 6 is a collection of three graphs showing the advantages of thethree stages provided by this invention. Each of these three graphs(FIG. 6) represents tests run in a relatively small ten and a quarterinch diameter column containing a catalyst bed with air and waterpassing through it. With a gas superficial velocity of 0.14 cubic feetper second (FIG. 4A), there was a reduction in gas holdup in the recycleline with the three stages especially as compared with the downcomerrecycle pan 88 used alone. FIG. 6B shows a more dramatic improvementwhen the gas flow was increased to 0.35 cubic feet per second. At 0.56cubic feet per second, there was a further improvement.

FIG. 7 shows a graph which illustrates elutriation in a column which wasten and a quarter inches in diameter, and charged with a conventionalcatalyst, water, and air, which were used as the three phases in thereactor. In the column, the height of the freeboard (free liquid abovethe catalyst bed) was 39-45 inches. This graph shows that elutriationwas approximately cut in half by the second stage and virtuallyeliminated by the third stage.

FIGS. 8A, 8B are the results of tests conducted on the inventivestructure in a pilot plant column which is four feet in diameter thatwas charged with catalyst, kerosene, and nitrogen.

FIG. 9 shows the dimensions of the four foot diameter pilot plant columnused for conducting the tests recorded in FIGS. 8A, 8B. The recycle pan88 was three feet in diameter with a wall height of twelve inches. Thevertical height of the second stage skirt 106 (FIG. 5) was eight inches.The vertical height of the third stage toroidal skirt 206 was sixinches. There was a four inch vertical separation between the apex 222of the third stage 206 and the lower most periphery of the second stage106. There was a two inch vertical difference between the level of innereave 224 and the level of outer eave 220. The outer gap 228 at thereactor wall 45 was varied from one to three inches for both of theskirts 106 and 206 (a three inch gap was used during the test reportedin FIGS. 8A, 8B). There was a six inch inner gap 226 between the innerperiphery of toroidal skirt 206 and the downcomer 78.

FIG. 8A gives the test results for a gas velocity of 0.18 cubic feet persecond, with a gas holdup in the order of about 40%. In each case, theaddition of the third stage gave an improvement which was judgedsubstantial. Further, it was found that, in general, decreasing theouter gap 228 between the vessel wall and the outer periphery of theskirts produced a significant improvement. The addition of the thirdstage 206 decreased gas holdup in the recycle line by up to 3 vol. %.

The foregoing description has explained a presently preferredembodiment. However, various modifications are immediately apparent tothose skilled in the art. For example, the skirts or baffles have beendescribed as being somewhat conical in cross-section. It should beunderstood that other shapes such as domes, inverted somewhat flatbottomed pans, or the like may also be used. The various surfaces shouldset at an angle which will cause any catalyst to slide off and not tocollect on the baffle itself.

Another modification is to provide still more baffles. There may be afourth, fifth . . . nth stage of baffles. Each baffle should be in thefreeboard zone and should not extend into the catalyst bed.

Those who are skilled in the art will readily perceive how to furthermodify the invention. Therefore, the appended claims are to be construedto cover all equivalent structures which fall within the true scope andspirit of the invention.

The claimed invention is:
 1. An ebullated bed reactor comprising anelongated closed vessel having a top and a bottom and including meansfor inputting hydrogen and oil into the bottom thereof, said vesselcontaining a catalytic bed which may be fluidized by a bottom feed ofoil and hydrogen which moves in an updraft through said bed in responseto its own buoyancy; downcomer means having a top and a bottom centrallylocated within said vessel, said downcomer means extending from alocation near the top of said vessel to a location near the bottom ofsaid vessel, ebullating pump means at the bottom of said downcomer meansfor recirculating at least some of said oil from the top of said vesselthrough said downcomer means and back into said catalytic bed, three gasseparation stages comprising a recycle collection pan, and twovertically deployed stationary flared annular frustoconical rigidskirts, means at the top and adjacent to the upper end of said downcomermeans for removing vapor and gas entrained in said oil, said downcomermeans having a predetermined inside diameter, a first of said stagesdefined by said recycle collection pan at said top of said downcomermeans, said recycle pan having an inside diameter which is larger thansaid predetermined inside diameter of said downcomer, said largerdiameter causing oil velocity and hydrogen motion to be reduced withinsaid recycle pan before said oil descends through said downcomer means,a second of said stages defined by one of two said frustoconical skirtsand being attached to and dependent from said recycle pan for collectingvapor and gas from said oil, and a third of said stages defined by asecond of said two annular skirts and having a bevel roof, said thirdstage being attached to and suspended below said second stage to collectgas from said updraft before it reaches said second stage.
 2. Thereactor of claim 1 wherein said third stage toroidal skirt surroundssaid downcomer means with a first annular gap between an insideperiphery of said toroid and said downcomer means, a second annular gapbetween an outside periphery of said toroid and a wall of said reactor,said first annular gap being larger than said second annular gap.
 3. Thereactor of any one of the claims 1 or 2 wherein said bevel roof has anapex ridge with one side sloping downwardly therefrom to eaves at aninside perimeter and another side sloping downwardly therefrom to eavesat an outside perimeter, the length of said one side being much lessthan the length of said other side.
 4. The reactor of claim 3 whereinsaid bevel roof toroid is positioned to cause liquid circulationpatterns around the toroidal skirt.
 5. The reactor of claim 4 includingmeans for venting each of said second and third stages to carry away thevapor and gas collected by said second and third stages.
 6. A reactorfor refining resid, said reactor comprising an elongated vesselcontaining catalyst particles, means for feeding a mixture of oil andgas into a plenum defined beneath said catalyst particles with enoughforce to create an ebullated catalyst bed and elutriate said particles,downcomer means for recycling said oil through said bed, liquid recyclepan means attached to an upper end of said downcomer means, means forwithdrawing a gas product from a level in said vessel which is higherthan said recycle pan means, and means for substantially precludingpassage of said elutriated particles into said downcomer means and saidproduct withdrawing means by creating a circulating current around saidprecluding means in a region surrounding said downcomer means and belowsaid recycle pan means, said current redirecting elutriated catalystdownwardly toward said catalyst bed, said precluding means supportedbelow the recycle pan means in proximity to said upper end andsurrounding said downcomer means.
 7. The reactor of claim 6 wherein saidprecluding means comprises a plurality of baffle means for entrapping,said elutriated particles into said downcomer means and said collecting,and venting said gas from said mixture.
 8. The reactor of claim 6wherein said precluding means comprises means for guiding and directingsaid elutriated particles out of said mixture and back to said catalystbed.